Summary

Field experiments were conducted during the period of 2004 – 2006 at the Research and Study Farm „Vecauce”. The aim of the investigation was to establish the weediness’ dynamics of crops in a six-field crop rotation of an organic farming system. The field was certified as being organic. Crop rotation: 1. triticale/ rye (for green manure in spring), 2. oil radish (for green manure)/ rye (for green manure in autumn), 3. potatoes, 4. oats, 5. vetch-oats mix (for green forage) with an undersown mixture of red clover and timothy, 6. red clover and timothy. Conventional soil tillage was done according to the requirements of the crops. The weeds were counted two times during the vegetation period of crops: at the early development stages of crops (for instance, at the tillering of cereals) and before the crop harvesting. The composition of weed flora comprised from 59 species in the crops’ stands. The annual dicotyledonus were dominating groups of the weed flora. The most abundant weeds were lambsquarters (Chenopodium album L.), quackgrass (Elytrigia repens (L.) Nevski), common yellow rocket (Barbarea vulgaris R. Br.p.p.). The significantly smaller amount of weeds among crop rotation links was observed in the link rye/ oil radish/ rye – potatoes – oats. Much worse suppression of weeds was obtained in the crop rotation link red clover and timothy – triticale/ rye – oil radish/ rye. The number of weeds in the fields of crop rotation in the organic farming system is dependent on the cultivated crops, the succession of crops in the crop rotation links and crop cultivation technologies.